The present invention is directed to ink compositions and printing processes. More specifically, the present invention is directed to ink compositions containing polymolecular micelles of a block copolymer having dye molecules attached to the micelle surfaces, and to printing processes employing said inks. One embodiment of the present invention is directed to an ink composition which comprises an aqueous liquid vehicle and particles of an average diameter of 100 nanometers or less which comprise micelles of block copolymers of the formula ABA, wherein A represents a hydrophilic segment and B represents a hydrophobic segment, and wherein dye molecules are covalently attached to the micelles. Optionally, silica is precipitated within the micelles. In another specific embodiment, the dye is selected so that the particles are detectable when exposed to radiation outside of the visible wavelength range. In yet another specific embodiment, the dye is selected so that the particles are substantially colorless and detectable when exposed to radiation outside of the visible wavelength range. The inks of the present invention are particularly suitable for ink jet printing processes.
Ink jet printing systems generally are of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field which adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium.
Since drop-on-demand systems require no ink recovery, charging, or deflection, they are much simpler than the continuous stream type. There are two types of drop-on-demand ink jet systems. One type of drop-on-demand system has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, and physical limitations of the transducer result in low ink drop velocity. Low drop velocity seriously diminishes tolerances for drop velocity variation and directionality, thus impacting the system's ability to produce high quality copies. Drop-on-demand systems which use piezoelectric devices to expel the droplets also suffer the disadvantage of a slow printing speed.
The second type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets and allows very close spacing of nozzles. The major components of this type of drop-on-demand system are an ink-filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle causing the ink in the immediate vicinity to evaporate almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands. When the hydrodynamic motion of the ink stops, the process is ready to start all over again. With the introduction of a droplet ejection system based upon thermally generated bubbles, commonly referred to as the "bubble jet" system, the drop-on-demand ink jet printers provide simpler, lower cost devices than their continuous stream counterparts, and yet have substantially the same high speed printing capability.
The operating sequence of the bubble jet system begins with a current pulse through the resistive layer in the ink filled channel, the resistive layer being in close proximity to the orifice or nozzle for that channel. Heat is transferred from the resistor to the ink. The ink becomes superheated far above its normal boiling point, and for water based ink, finally reaches the critical temperature for bubble formation or nucleation of around 280.degree. C. Once nucleated, the bubble or water vapor thermally isolates the ink from the heater and no further heat can be applied to the ink. This bubble expands until all the heat stored in the ink in excess of the normal boiling point diffuses away or is used to convert liquid to vapor, which removes heat due to heat of vaporization. The expansion of the bubble forces a droplet of ink out of the nozzle, and once the excess heat is removed, the bubble collapses on the resistor. At this point, the resistor is no longer being heated because the current pulse has passed and, concurrently with the bubble collapse, the droplet is propelled at a high rate of speed in a direction towards a recording medium. The resistive layer encounters a severe cavitational force by the collapse of the bubble, which tends to erode it. Subsequently, the ink channel refills by capillary action. This entire bubble formation and collapse sequence occurs in about 10 microseconds. The channel can be refired after 100 to 500 microseconds minimum dwell time to enable the channel to be refilled and to enable the dynamic refilling factors to become somewhat dampened. Thermal ink jet processes are well known and are described, for example, in U.S. Pat. Nos. 4,601,777, 4,251,824, 4,410,899, 4,412,224, and 4,532,530, the disclosures of each of which are totally incorporated herein by reference.
Known ink jet inks generally comprise a water soluble dye which is soluble in an ink vehicle such as water or a mixture comprising water and a water soluble or water miscible organic solvent. Inks comprising soluble dyes may exhibit many problems, such as poor waterfastness, poor lightfastness, clogging of the jetting channels as a result of solvent evaporation and changes in the solubility of the dye, dye crystallization, ink bleeding when prints are formed on plain papers, poor thermal stability, chemical instability, ease of oxidation, and low drop velocity. In addition, many of the dyes contained in inks may be potentially toxic or mutagenic. These problems can be minimized by replacing the dyes used in ink formulations with insoluble pigments. In general, pigments are superior to dyes with respect to waterfastness, lightfastness, image density, thermal stability, oxidative stability, compatibility with both coated/treated and plain papers, image edge acuity, reduced image feathering, and non-toxic and non-mutagenic properties.
Heterophase ink jet inks are known. For example, U.S. Pat. No. 4,705,567 discloses a heterophase ink jet ink composition which comprises water and a dye covalently attached to a component selected from the group consisting of poly(ethylene glycols) and poly(ethylene imines), which component is complexed with a heteropolyanion. In addition, U.S. Pat. No. 4,597,794 discloses an ink jet recording process which comprises forming droplets of an ink and recording on an image receiving material by using the droplets, wherein the ink is prepared by dispersing fine particles of a pigment into an aqueous dispersion medium containing a polymer having both a hydrophilic and a hydrophobic construction portion. The hydrophilic portion constitutes a polymer of monomers having mainly polymerizable vinyl groups into which hydrophilic portions such as carboxylic acid groups, sulfonic acid groups, sulfate groups, and the like are introduced. Pigment particle size may be from several microns to several hundred microns. The ink compositions disclosed may also include additives such as surfactants, salts, resins, and dyes.
U.S. Pat. No. 4,877,451 (Winnik et al.), the disclosure of which is totally incorporated herein by reference, discloses ink jet ink compositions comprising water, a solvent, and a plurality of colored particles comprising hydrophilic porous silica particles to the surfaces of which dyes are covalently bonded through silane coupling agents. In addition, copending application U.S. Ser. No. 07/369,003, the disclosure of which is totally incorporated herein by reference, discloses ink jet inks and liquid developers containing colored particles comprising hydrophilic porous silica particles to the surfaces of which dyes are covalently bonded through silane coupling agents.
Heterophase inks containing pigment particles as colorants, however, also exhibit difficulties. For example, the particulate colorant may exhibit a tendency to settle out or separate from the liquid vehicle, particularly when the ink is stored for long periods of time. In addition, inks containing pigment particles as colorants tend to be opaque instead of transparent, which reduces their usefulness for printing images on transparencies for the purpose of overhead projection. Further, inks containing pigment particles as colorants tend to clog the narrow orifices of the printhead, resulting in deterioration of the print quality.
Accordingly, there is a need for inks having the advantages of both dye-based inks and pigment-based inks. The present invention is directed to an ink composition which comprises a liquid vehicle and colored particles of an average diameter of 100 nanometers or less which comprises micelles of block copolymers of the formula ABA, wherein A represents a hydrophilic segment and B represents a hydrophobic segment, and wherein dye molecules are covalently attached to the micelles. Optionally, silica is precipitated within the micelles.
U.S. Pat. No. 4,552,914 (Sterling) discloses a thermoplastic elastomeric hydrocarbon block copolymer having 0.1 to 8 percent polysiloxane or silicone oil uniformly distributed throughout. The block copolymer may be styrene-ethylene-butylene-styrene in which the styrene blocks have a molecular weight of 5,000 to 40,000 and the ethylenebutylene block has a molecular weight of 20,000 to 500,000. The copolymer is combined with the silicone oil under the influence of a pressure of 1,500 to 2,500 p.s.i. such as provided by extrusion blending. The composition exhibits improved surface, elasticity, and tensile strength characteristics as well as superior processability.
U.S. Pat. No. 4,267,310 (Landoll) discloses a process for producing uniformly spherical particles of crystalline, normally solid condensation polymers by forming a dispersion of small liquid particles of the crystalline polymer in aprotic liquids containing 0.1 to 20 percent by weight of the polymer of a water-insoluble block or graft copolymer having at least one polymeric component which is soluble in the aprotic liquid and at least one other polymeric component which is insoluble in the aprotic liquid and associates with the crystalline polymer. The dispersion is cooled with agitation until the liquid particles solidfy, and the particles are then recovered.
U.S. Pat. No. 4,153,587 (Yui) discloses a modified propylene polymer containing mixed polymer composition characterized by high tensile strength, high elongation, and good bending stiffness. The material consists essentially of a crystalline polypropylene, a block copolymer wherein a propylene-ethylene random copolymer is chain-terminated onto the crystalline polypropylene, and a crystalline ethylene-propylene random copolymer having an ethylene content of more than about 90 percent by weight, wherein the modified polypropylene polymer is admixed with at least 30 percent by weight of an inorganic filler having an average particle size of less than 20 microns in diameter.
Copending application U.S. Ser. No. 07/636,264, entitled "Method of Storing Information Within a Reprographic System," with the named inventor Joseph D. Wright, the disclosure of which is totally incorporated herein by reference, discloses apparatuses and processes for controlling a reproduction system by scanning an image to detect at least one taggant in at least one marking material forming the image and issuing instructions to the reproduction system; the instructions cause the reproduction system to take an action selected from the group consisting of (a) prohibiting reproduction of those portions of the image formed by a marking material containing at least one predetermined detected taggant and reproducing all other portions of the image; (b) prohibiting reproduction of any part of the image upon detection of at least one predetermined taggant; (c) reproducing only those portions of the image formed by a marking material containing at least one predetermined taggant; (d) reproducing portions of the image formed by a marking material containing at least one predetermined taggant in a different manner from that in which the system reproduces portions of the image formed by a marking material not containing at least one predetermined taggant; and (e) identifying a source of the image on the basis of detection of at least one predetermined taggant.
Although known compositions are suitable for their intended purposes, a need remains for ink compositions exhibiting advantages of both dye-based inks and pigment-based inks. There is also a need for ink compositions with good waterfastness characteristics. A need also remains for ink compositions exhibiting good lightfastness characteristics. Further, there is a need for ink compositions that are non-toxic and non-mutagenic. In addition, a need exists for ink compositions for which a wide variety of color choices exists. There is also a need for ink compositions that can be prepared by simple and inexpensive processes. A need also exists for ink compositions which comprise stable particulate suspensions with long shelf life. Further, there is a need for ink compositions suitable for printing on plain papers, coated or treated papers, and transparency materials. In addition, there is a need for ink compositions that when printed on substrates exhibit good optical density, low feathering, and excellent rubresistance. There is also a need for ink compositions that when used to print on transparency materials generate images that project their original colors well when light is passed through the image. A need also remains for ink compositions with good thermal and oxidative stability. Further, there is a need for ink compositions suitable for ink jet printing that do not induce clogging of the printhead. A need also remains for ink compositions that when printed on substrates exhibit no undesirable intercolor bleeding between areas of different color. In addition, there is a need for ink compositions that are invisible to the human eye under normal viewing conditions but readable by a sensor, such as an infrared detector or a fluorescence detector, or by the human eye under special viewing conditions such as illumination of the image with ultraviolet light. There is also a need for ink compositions that are visible to the human eye under normal viewing conditions and are also readable by a sensor that detects wavelengths invisible to the human eye, such as an infrared detector or a fluorescence detector, or by the human eye under special viewing conditions such as illumination of the image with ultraviolet light. Further, there is a need for ink compositions that can provide a means for placing coded information on a document.